Ségolène Royal, the French Socialist party's presidential favourite, was forced on to the defensive on Friday when she was booed for the first time in a public debate and an opinion poll suggested she was losing ground to her two rivals.

"Socialists should not jeer each other. I don't understand what happened," she said after being forced to speak over boos and whistles from part of the crowd during a party debate in Paris.

Analysts, however, said the humbling experience could turn out to benefit Ms Royal in the long run as it was likely to bolster her image as an outsider from the Paris party machine, which was likely to go down well with party members in the provinces.

"I hope this will be the first and last time that this happens during this long campaign. I think everyone has to calm their troops," said Ms Royal after the debate at Zenith concert hall in Paris on Thursday night.

Many members of the Paris section of the Socialist party are thought to favour Dominique Strauss-Kahn, the former finance minister, who is a deputy for the nearby Sarcelles constituency and, as an economics professor, appeals to their more urban and intellectual interests.

Mr Strauss-Kahn was the most warmly applauded of the three candidates. ­Laurent Fabius, former prime minister and flag-bearer for the left wing of the party, also suffered some booing, but not as much as Ms Royal.

One adviser to Mr Strauss-Kahn admitted the vocal opposition to Ms Royal had been organised in advance and was not based on anything she said at the debate.

However, the crowd seemed particularly opposed to her recent proposal to introduce "citizen juries" to judge how well politicians were performing between elections, which has been widely attacked as dangerously populist.

With three weeks to go until the 200,000 party members vote on their candidate, there are growing signs that the euphoria surrounding Ms Royal's candidacy is starting to wear off.

Pierre Giacometti, head of polling agency Ipsos, said: "She has won the image battle and shown she is very good at this. But on her ideological positions, she knows that there is a large part of the Socialist party that is very shocked by some of her ideas."

An opinion poll by CSA published in yesterday's Bleu de Profession Politique showed that among 308 Socialist supporters, Ms Royal had fallen from 72 per cent to 57 per cent in a week.

But Jean-Daniel Levy, head of research at CSA, played down the poll's findings, saying: "She is still far ahead of the others. I tend to think that this is a small air pocket and the reasons people support Ségolène Royal still exist."